The present invention relates to tuned channel detection systems and, more particularly, to television signal detection methods and apparatus for determining the channel to which a television receiver is tuned.
For marketing research, program ratings, consumer surveys, and the like, it is often advantageous to determine the channels to which the televisions within a given viewing area are tuned. The motivation and desire for collecting such information is well-known and thus, further elaboration is unnecessary.
Conventional methods for determining the channel to which a television (TV) receiver is tuned involve the detection of the TV's local oscillator signal. The detection of the local oscillator signal in and of itself is old and well-known in the art. The prior art illustrates various apparatus, both invasive and non-invasive to the television receiver circuitry, which have been used as attempts to provide channel detection means which are more robust and less susceptible to a false reading. Enhanced methods are also used in the detection of the TV's local oscillator signal. Non-invasive methods have typically used an antenna tuned to the TV's local oscillator signal, thus no direct physical connection is made to the television. Invasive techniques, on the other hand, typically use a probe to a circuit point within a TV's tuner circuitry or within a set top cable converter box which provides the detector apparatus with a direct connection to the tuner, thus allowing for both the injection and measurement of signals at the tuner or set top converter. While a direct connection makes it easier to detect the local oscillator signal, it is obviously more desirable to use more non-invasive approaches.
Some examples of non-invasive methods for improving the integrity of local oscillator signal detection include such systems as those disclosed in U.S. Pat. No. 4,723,302 issued Feb. 2, 1988 to Fulmer, et al.; U.S. Pat. No. 3,312,900 issued Apr. 4, 1967 to Jaffe; and U.S. Pat. No. 4,577,220 to Laxton, et al.
Fulmer, et al. describe detecting the local oscillator signal of the TV and storing characteristic values of the signal for the fundamental and a plurality of harmonic frequencies of the local oscillator signals which correspond to predetermined channels. The local oscillator signal fundamental frequency and the corresponding harmonic frequencies which are observed are compared to the stored values to identify the tuned channel. The Fulmer system uses an antenna tuned to the local oscillator or, in the alternative, a direct connection to the radio frequency (RF) input cable signal path.
The Jaffe and Laxton systems both use an antenna tuned to the local oscillator signal and placed in the vicinity of the tuner circuitry of the TV set. Since signals from the television line scanning circuitry tend to modulate the local oscillator signal, the Jaffe system extracts the line scanning information from the local oscillator signal to identify the tuned channel. The Laxton system uses a closed loop arrangement to "lock on" the frequency of the detected local oscillator signal.
Notwithstanding the methods and apparatus described thus far, tuned channel detection through the determination of a TV receiver's local oscillator signal frequency remains problematic. The tuners used by the various manufacturers of TVs, video cassette recorders (VCRs) and cable converter boxes (set top converters) naturally have varying characteristics thus making the positioning of an antenna appropriately in the vicinity of the tuner problematic. Moreover, the frequency of the local oscillator signals generated within TVs, VCRs, and set top converters will range, depending upon the tuned channel, from about 100 MHz to 1400 MHz which in and of itself makes detection a considerable task. In any case, the local oscillator signal's location will only be known approximately, and typically it will be a very low level signal buried down in the noise. It may also be difficult to discern local oscillator from the color carrier when they are in close proximity. These factors along with signal interference from other sources make it desirable to provide a tuned channel detection means having greater integrity than that provided by today's systems. To this end, better non-invasive methods for detecting and/or verifying the tuned channel including improved methods of detecting local oscillator signals are desirable.